How to flip the view from the camera horizontally?
You can scale the camera negatively along it's local 'x' axis to acheive a flipped view (while looking through the camera).

Select the camera and press S,X,X,-,1

Pressing X twice will scale along the 'local' axis instead of the 'global' axis. 

Alternatively just enter '-1' into the 'x' scale in the object panel: